1. Field of the Invention
This invention relates to a power supply antenna and a power supply method. More specifically, the invention relates to a power supply antenna which is useful for a plasma.
2. Description of the Related Art
In the field of semiconductor manufacturing, film formation using a plasma assisted chemical vapor deposition (plasma CVD) system is currently known. The plasma CVD system is designed to introduce a starting gas, which will be materials of a film, into a deposition chamber inside a vessel to convert it into the state of a plasma, and promote a chemical reaction on the surface of a substrate by active excited atoms or molecules in the plasma to deposit a film. To create the plasma state in the deposition chamber, the vessel is provided with an electromagnetic wave transparent window, and a power supply antenna located outside the vessel is supplied with an electric power to enter an electromagnetic wave through the electromagnetic wave transparent window.
FIG. 11 is a view showing a power supply antenna according to an earlier technology, which is used in the above-described semiconductor manufacturing apparatus. As shown in this drawing, a power supply antenna 01 is a single loop antenna with a single power supply portion 01A. This power supply antenna 01 is usually disposed at the top of a cylindrical vacuum vessel 02 so as to convert a gas, which has been injected into the vacuum vessel 02, into a plasma, thereby depositing a film on a wafer 04 borne on an electrostatic chuck 03 and disposed below. If cylindrical coordinates with the center of the wafer 04 as an origin O are assumed, a coordinate axis r represents a radial direction, a coordinate axis Z represents a cylindrical axial direction, and θ represents a circumferential direction.
With the single loop antenna having the power supply portion 01A at one location, as described above, the value of an electric current flowing through each part of the power supply antenna 01 is, needless to say, constant. In such a current distribution, distribution of absorption (in a radial direction), by plasma, of an electromagnetic wave from the power supply antenna 01 shows marked nonuniformity. FIG. 12 shows the electromagnetic wave energy absorption distribution of plasma determined by numerically finding the propagation in the plasma of the electromagnetic wave (i.e., solving a wave equation of the electromagnetic wave) from the power supply antenna 01. The horizontal axis of FIG. 12 represents the position (m) in the diametrical direction relative to the origin as the center of the power supply antenna 01 (origin O as the center of the wafer 04). The vertical axis represents the amount of absorption of the electromagnetic wave energy (W/m3). The characteristics of a solid line in FIG. 12 show an absorbed power distribution at the position 0.16 (m) vertically (in the Z direction) above the surface of the wafer 04 illustrated in FIG. 11. Z=0.16 means this fact (the same will be true of the description to follow) As will be seen in FIG. 12, strong peaks appear around points corresponding to a half of the radius of the vacuum vessel 02, and energy absorptions are very weak at the center and on the periphery of the vacuum vessel 02. In a region near the center and distant from the wall of the vacuum vessel 02, the plasma diffuses toward the center where the temperature and the density are low, and the distribution of the diffusing plasma relatively flattens over time. In a peripheral region close to the wall, the plasma escapes to this wall. Thus, the plasma cannot be flattened in the peripheral region. As a result, the temperature and density of the plasma are low in the peripheral region. Hence, film deposition cannot ensure the uniformity of the film thickness throughout the surface of the wafer 04. This is confirmed experimentally.